What happens?

What happens when you mix Euro coins and batteries in your back pocket, as I did in my jeans today? Well, interestingly, as I was sitting in my sister's car while we were going to visit mom, my right butt cheek started feeling hot, then almost burning. What was going on??

I pulled out the coins and batteries and found that two of the batteries had become very hot! So the contact between those batteries and coins were causing some kind of overload and heating them up fast. If I had left them for much longer, I wonder if they might not have exploded. They always tell you not to throw batteries in a fire for fear that they will explode.

Any chemical engineers out there want to clue us in?